The present invention relates to a preform for use in making a video disc record and the method of making a disc therefrom and more particularly to such a preform which results in the reduction of such defects as voids, blisters and pits on the surface of the disc.
Disc records are typically made by placing a plastic preform centrally between a pair of heated molds. The molds are closed against the preform causing the plastic to flow radially outwardly to fill the mold cavity and define the record contour. The preforms heretofore used had flat top and bottom surfaces extending completely across the peripheral surface, which was generally cylindrical. A problem with this type of preform is that the flat surfaces are not completely smooth but contain small recesses or cavities. This results in air and gasses being trapped between the flat surfaces of the preform and the surfaces of the mold within these recesses or cavities. The trapped air and gasses cannot escape as the molds are closed and placed against the preform so that they cause the formation of such defects as voids, blisters and pits in the record surface. Such defects can result in misformed grooves which can cause stylus mistracking. Such defects are even a greater problem and therefore especially undesirable in high density information records, such as video discs, which contain a groove packing density in the order of 10,000 grooves per inch.